A question such as this one can support a wide range of topics for discussion. For example, this essay can discuss the role of geography in shaping history, given the degree to which Greece's own highly mountainous terrain proved a severe barrier to unification, influencing the evolution of the various city-states. At the same time, you can discuss how the Greeks relied on the Mediterranean Sea when it came to commerce and travel while also using it as an avenue for expansion, with the Greek city-states founding colonies across the larger Mediterranean world.
There are other approaches as well. You might discuss the history of ancient Greece from an extremely long-term perspective. In this case, you might stretch the timeline back to the ancient Mycenaeans, discussing the fall of Mycenaean civilization and the advent of the Greek Dark Age (which provided the context from which Hellenic civilization would eventually emerge).
At the same time, you might focus your essay specifically on the history of classical Greece. Such an essay might focus on the contrasting examples of Sparta and Athens (the two most famous Greek city-states), or it might look at the ancient Greeks within the context of the larger Mediterranean world, discussing the impact and influence of other cultures and civilizations within this history.
To conclude, there are a lot of directions this essay can be taken, and a lot that might be worth thinking about.